For example, in a production line of automobiles, there is a case where, to an unpainted vehicle body conveyed by a supporting conveyance line on the floor side constituted by a floor conveyor such as a carriage type conveyor or a slat conveyor, an unpainted door conveyed while being supported by a hanger in a hanging conveyance line constituted by an overhead type trolley conveyor or the like, etc., is assembled, and conveyed to a painting line, and a case where, to remove a door from a painted door-attached vehicle body and convey the vehicle body and the door to mutually different lines, a door removed from a door-attached vehicle body conveyed by the supporting conveyance line is loaded onto the hanging conveyance line and conveyed. As an object loading and unloading conveyance device to be used in such cases, for example, as described in Japanese Patent Publication No. 2890216 (Japanese Unexamined Patent Publication No. H05-685), there is known an object loading and unloading conveyance device which includes a hanging conveyance line in which a pallet which supports an object (door) is supported and conveyed by a hanger and a supporting conveyance line in which an object (vehicle body) is supported and conveyed below the hanging conveyance line, and conveys the pallet between a hanger stop position of the hanging conveyance line and an object loading and unloading position lateral to the supporting conveyance line, comprised of a horizontal conveyance device which conveys the pallet substantially horizontally between a pallet transferring position set above the object loading and unloading position and a hanger stop position in the hanging conveyance line, and an elevating conveyance device which conveys the pallet vertically between the object loading and unloading position and the pallet transferring position.
However, the conventional conveyance device as described in the aforementioned patent publication, etc., uses an elevating conveyance device of a type called a drop lifter comprising one pallet mounting base that reciprocates on one side of elevating guide rails so as to move linearly vertically. Therefore, to assemble a door conveyed by the overhead hanging conveyance line to a vehicle body to be conveyed by the supporting conveyance line on the floor side, an operator manually extracts the door from a door mounting pallet sent to an object loading and unloading position by the side of the supporting conveyance line on the floor side, and thereafter, a subsequent door mounting pallet must be made to wait in the hanging conveyance line unless the empty pallet is returned to the hanger of the hanging conveyance line from which the door mounting pallet was unloaded and is waiting in the empty state by the elevating conveyance device and the horizontal conveyance device. That is, one cycle time during which a door mounting pallet is put down from the hanger conveyed to a predetermined position (hanger stop position) by the hanging conveyance line, to a predetermined position (object loading and unloading position) and the door is extracted from the hanger, and the empty pallet is returned to the original hanger of the hanging conveyance line that is stopped and waits, and a subsequent door mounting pallet is sent to a predetermined position (hanger stop position), becomes very long, and no matter how quickly an operator performs the work of assembling the door extracted from the door mounting pallet to a vehicle body on the supporting conveyance line on the floor side, the working efficiency of the whole conveyance device cannot be improved.